Malaria currently represents one of the most prevalent infections in tropical and subtropical areas throughout the world. Per year, malaria infections lead to severe illnesses in hundreds of million individuals worldwide, while it kills 1 to 3 million people, primarily in developing and emerging countries every year. The widespread occurrence and elevated incidence of malaria are a consequence of the increasing numbers of drug-resistant parasites and insecticide-resistant parasite vectors. Other factors include environmental and climatic changes, civil disturbances, and increased mobility of populations.
Malaria is caused by the mosquito-borne hematoprotozoan parasites belonging to the genus Plasmodium. Four species of Plasmodium protozoa (P. falciparum, P. vivax, P. ovale and P. malariae) are responsible for the disease in man; many others cause disease in animals, such as P. yoelii and P. berghei in mice. P. falciparum accounts for the majority of infections and is the most lethal type (“tropical malaria”). Malaria parasites have a life cycle consisting of several stages. Each stage is able to induce specific immune responses directed against the corresponding occurring stage-specific antigens.
Malaria parasites are transmitted to man by several species of female Anopheles mosquitoes. Infected mosquitoes inject the “sporozoite” form of the malaria parasite into the mammalian bloodstream. Sporozoites remain for a few minutes in the circulation before invading hepatocytes. At this stage, the parasite is located in the extra-cellular environment and is exposed to antibody attack, mainly directed to the “circumsporozoite” (CS) protein, a major component of the sporozoite surface. Once in the liver, the parasites replicate and develop into so-called “schizonts.” These schizonts occur in a ratio of up to 20,000 per infected cell. During this intra-cellular stage of the parasite, main players of the host immune response are T-lymphocytes, especially CD8+ T-lymphocytes (Romero et al. 1998). After about one week of liver infection, thousands of so-called “merozoites” are released into the bloodstream and enter red blood cells, becoming targets of antibody-mediated immune response and T-cell secreted cytokines. After invading erythrocytes, the merozoites undergo several stages of replication and transform into so-called “trophozoites” and into schizonts and merozoites, which can infect new red blood cells. This stage is associated with overt clinical disease. A limited amount of trophozoites may evolve into “gametocytes,” which is the parasite's sexual stage. When susceptible mosquitoes ingest erythrocytes, gametocytes are released from the erythrocytes, resulting in several male gametocytes and one female gametocyte. The fertilization of these gametes leads to zygote formation and subsequent transformation into ookinetes, then into oocysts, and finally into salivary gland sporozoites.
Targeting antibodies against gametocyte stage-specific surface antigens can block this cycle within the mosquito mid gut. Such antibodies will not protect the mammalian host but will reduce malaria transmission by decreasing the number of infected mosquitoes and their parasite load.
Current approaches to malaria vaccine development can be classified according to the different stages in which the parasite can exist, as described above. Three types of possible vaccines can be distinguished:                Pre-erythrocytic vaccines, which are directed against sporozoites and/or schizont-infected cells. These types of vaccines are primarily CS-based, and should ideally confer sterile immunity, mediated by humoral and cellular immune response, preventing malaria infection.        Asexual blood-stage vaccines, which are designed to minimize clinical severity. These vaccines should reduce morbidity and mortality and are meant to prevent the parasite from entering and/or developing in the erythrocytes.        Transmission-blocking vaccines, which are designed to hamper the parasite development in the mosquito host. This type of vaccine should favor the reduction of population-wide malaria infection rates.        
Next to these vaccines, the feasibility of developing malaria vaccines that target multiple stages of the parasite life cycle is being pursued in so-called multi-component and/or multi-stage vaccines. Currently, no commercially available vaccine against malaria is available, although the development of vaccines against malaria has already been initiated more than 30 years ago: immunization of rodents, non-human primates and humans with radiation-attenuated sporozoites conferred protection against a subsequent challenge with sporozoites (Nussenzweig et al. 1967; Clyde et al. 1973). However, the lack of a feasible large-scale culture system for the production of sporozoites prevents the widespread application of such vaccines.
To date, the most promising vaccine candidates tested in humans have been based on a small number of sporozoite surface antigens. The CS protein is the only P. falciparum antigen demonstrated to consistently prevent malaria when used as the basis of active immunization in humans against mosquito-borne infection, albeit it at levels that is often insufficient. Theoretical analysis has indicated that the vaccine coverage, as well as the vaccine efficiency, should be above 85% or, otherwise, mutants that are more virulent may escape (Gandon et al. 2001).
One way of inducing an immune response in a mammal is by administering an infectious carrier that harbors the antigenic determinant in its genome. One such carrier is a recombinant adenovirus, which has been replication-defective by removal of regions within the genome that are normally essential for replication, such as the E1 region. Examples of recombinant adenoviruses that comprise genes encoding antigens are known in the art (PCT International Patent Publication WO 96/39178), for instance, HIV-derived antigenic components have been demonstrated to yield an immune response if delivered by recombinant adenoviruses (WO 01/02607 and WO 02/22080). Also for malaria, recombinant adenovirus-based vaccines have been developed. These vectors express the entire CS protein of P. yoelii, which is a mouse-specific parasite, and these vectors have been shown to be capable of inducing sterile immunity in mice in response to a single immunizing dose (Brũna-Romero et al. 2001a). Furthermore, a similar vaccine vector using CS from P. berghei was recently shown to elicit long-lasting protection when used in a prime-boost regimen, in combination with a recombinant vaccinia virus (Gilbert et al. 2002) in mice. It has been demonstrated that CD8+ T-cells primarily mediate the adenovirus-induced protection. It is unlikely the P. yoelii- and P. berghei-based adenoviral vectors would work well in humans, since the most dramatic malaria-related illnesses in humans are not caused by these two parasites. Moreover, it is preferred to have a vaccine which is potent enough to generate long-lasting protection after one round of vaccination, instead of multiple vaccination rounds using either naked DNA injections and/or vaccinia-based vaccines as boosting or priming agents.
Despite all efforts to generate a vaccine that induces an immune response against a malaria antigenic determinant and protects from illnesses caused by the malaria parasite, many vaccines do not fulfill all requirements as described above. Whereas some vaccines fail to give a protective efficiency of over 85% in vaccinated individuals, others perform poorly in areas, such as, production or delivery to the correct cells of the host immune system.